Malignant progression of cancer is associated with increased resistance of neoplastic cells to existing therapies, and is responsible for the majority of the >40,000 breast-cancer-related yearly fatalities in the US alone. A better understanding of the malignant features of breast cancer cells (BCCs) and how they become metastatic will lead to the development of more effective therapies that save patients' lives. Our prior studies have identified that the microenvironment of breast tumors is enriched for certain progenitor cells called mesenchymal stem cells (MSCs), cells that otherwise contribute to the maintenance and regeneration of connective tissues during wounding and inflammation. Importantly, we demonstrated that tumor-associated MSCs exerted potent pro-malignant functions, causing even poorly metastatic breast cancer cells (BCCs) to spread to distant tissues. These findings ascribed a novel role for MSCs as important determinants of breast cancer pathogenesis and provided new insights into cancer metastasis. To elucidate the heterotypic interactions that MSCs establish with BCCs in the context of metastasis, we conducted detailed molecular analyses of MSC-stimulated BCCs, and identified miR-199a~214 as the most MSC-stimulated microRNA in BCCs. We present evidence that miR-199a~214 promotes cancer stem cell (CSC) -like properties in cancer cells, and that its actions involve the downregulation of FOXP2, a forkhead transcription factor tightly associated with speech and language development. We show that FOXP2 knockdown in BCCs phenocopies miR-199a~214 expression, and that it is sufficient in promoting CSC propagation, tumor-initiation, and metastasis. Importantly, we show that elevated miR-199a and depressed FOXP2 expression levels represent prominent features of malignant clinical breast cancer, associating significantly with triple-negative (TN) and HER2-enriched breast cancers. In this proposal, we will elucidate the involvement of the newly discovered miR-199a-FOXP2 axis in breast cancer pathogenesis. Using in vitro, in vivo, and clinical approaches, we will: (1) determine the role of miR- 199a as a biomarker of advanced clinical disease, and demonstrate its essentiality for metastasis in pre-clinical models so as to establish its relevance as a target in breast cancer therapy; (2) elucidate the metastasis- associated activities of miR-199a~214 in vitro and in vivo; and (3) decipher the regulatory molecular networks underlying the induction of miR-199a in BCCs, elucidate how such networks converge on and regulate FOXP2 silencing, and determine how FOXP2 exerts its downstream functions. Collectively, our proposed studies will identify and define novel and important molecular determinants that regulate breast CSC genesis, maintenance, and metastasis. These insights would shed light on the inner workings of breast cancer's most malignant cells, and will serve to provide novel tools of potential utility in the prognosis and therapy of malignant disease.